A light-emitting element using an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at low voltage, is expected to be used in a next-generation flat panel display In particular, a display device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
The light emission mechanism is said to be as follows: when a voltage is applied between a pair of electrodes with an EL layer including a luminous body provided therebetween, carriers (electrons and holes) are injected from the electrodes, and the carriers recombine to form excitons, and energy is released and light is emitted when the excitons return to the ground state. A singlet excited state (S*) and a triplet excited state (T*) are known as excited states. Light emission from the singlet excited state is called fluorescence and light emission from the triplet excited state is called phosphorescence. The statistical generation ratio of the excited states in a light-emitting element is considered to be S*:T*=1:3.
To improve element characteristics of such a light-emitting element, a light-emitting element containing a phosphorescent substance in which intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs has been actively developed. Furthermore, a light-emitting element in which layers containing different phosphorescent substances are stacked for white light emission is disclosed (see, for example, Patent Document 1).